Free piston engine



July 6, 1937. JUVNKERS' 2,086,163

FREE PI STON ENG INE Filed Jan. 9, 1954 2 Sheets-Sheet 1 c' c" a I! m M July 6, 1937. H. JUNKER 2,086,163

FREE PISTON ENGINE Filed Jan. 9, 1934 2 Sheets-Sheet 2 Fzlgt 3 7 O a O 7 Inventor.-

7 can be utilized in any suitable manner.

Patented July 6, 1937 PATENT OFFICE mar-z rrs'ron ENGINE Hugo Junkers, Dessau, Germany; Therese Junkers, nee Bennhold, Gauting, Germany, administratrix of said Hugo Junkers, deceased Application January 9, 1934, Serial No. 705,906

Germany January 18, 1933 7 Claims.

lviy invention relates to free piston engines in which a free piston or a pair 01' such pistons is or are arranged in a cylinder for reciproca-.

tion. It is an object'of my invention to improve the eiiiciency of this type of engines.

In free piston engines a mixture of fuel and air introduced into the space on one side of the piston, if only a single piston is provided, or into the space between the pistons if two pistons are provided (these spaces being referredto hereinafter as combustion chambers) and ignited causes the piston or pistons to move outwardly and to compress bodies of gas provided in the end portion or portions of the cylinder (referred to hereinafter as compression chambers) thereby transmitting to the gas energy.which In order to return each piston to its inner dead centre positions, it is necessary to, feed part of this energy back to the piston. The stroke oi the piston is not constant, but is varied either unintentionally, for instance when the friction between the piston and the wall of the cylinder is altered, or intentionally in order to control the delivery of the engine.

It is an object of my invention to operate a free piston engine in such manner that the inner dead centre positions of the pistons are not altered by variations of the stroke of the pistons, so that the compression of the charge in the combustion chamber is independent of the stroke of the piston or pistons and the driving part of the engine operates in the most efficient manner. Under these conditions a variation of the stroke of the piston merely affects the outer dead centre position of each piston, the inner dead centre position remaining unaltered. In order to accomplish this, it is necessary that the total amount of energy fed back by the gas to the pistons during their return stroke hereinafter called feed-back energy be substantially constant and independent of the stroke.

If this amount of feed back energy wereto also be compressed far more and during the combustion to follow the pressure and temperature would rise correspondingly, far beyond the normal, so that the motor would be endangered.

In order to maintain the feed-back energy constant and independent of thestroke, it has been proposed to provide separate gas-filled buffer cylinders which do not transmit energy from the engine to a consumer, but merely store energy during the working stroke and feed it back to the piston during its return stroke. When using such buifer cylinders, a resultant feed back energy, which remains substantially constant in spite of variations of stroke, would be obtained onlyv in the case where that part of the feed back energy, which is generated in the compressor cylinders, increases, while the stroke decreases.

However, as shown farther below, this will be the case only if the compressor operates with a high ratio of compression (feed pressure suction pressure). The addition of such separate buffer cylinders increases the number of parts in the engine and the space required for it. For

this latter reason, one always tries to keep these buifers as small as possible and on the other hand to operate them with high pressures. However operation under high pressures impairs the safety of operation of the machine, since in consequence of the high compression the gas in the; buffers is'highly heated, so that the parts are subjected to high heat stresses.

It is an object of my invention to provide an improved free piston engine in which the feedback energy is substantially constant and independent of the stroke of the piston or pistons, whereby buffers tobe separately added or which must be operated under high pressure can be dispensed with.

In order that my invention may be fully understood, reference is had to the accompanying drawings forming part of this specification and showing by way of example some embodiments of free piston engines according to my invention and some diagrams explaining the operation of the engines.

In the drawings: Figs. 1-3 are diagrams serving for the explanation of the operation of this engine, Figs. 4 and 5 are axial sections of two embodiments of an engine according to my invention.

Referring first to Fig. 1, in which the abscissae designate the volume v, the ordinates the pressure 12 of the gas in the compression chamber, be it assumed that m and m are the pressure and the volume respectively of the gas in the compression chamber at the inner dead centre position of the piston, while 112 is the pressure in the compression chamber at the outer dead centre position of the piston. Fig. 1 illustrates the volume-pressure diagrams for two strokes having different lengths and equal inner dead centre positions. At the beginning of the outward stroke of the piston the pressure and the volume are equal to m and m, respectively, this condition being indicated by the point A. During the first portion of the outward stroke the compression of the gas in the compression chamber is effected along the curve A, B, the pressure rising to the value 11:. Now compressed gas is expelled from the compression chamber till the piston reaches its outer dead centre position, the corresponding curve being a horizontal straight line B, C. C corresponds to the outer dead centre position of the piston and the first portion of the return stroke is effected along the curve C, D, the pressure dropping to 111 when the point D is reached. During the last portion of the return stroke corresponding to the horizontal straight line D, A fresh gas or air is sucked into the compression chamber. Dr and 124 are the volumes corresponding to the points C and D, respectively. The feed-back energy can be calculated from this diagram and is equal to the area of the surface G, A, D, C, E, G.

For a shorter stroke of the piston the diagram consists of the curve A, B, C, D and the feedback energy corresponding to this stroke is given by the area of the surface G, A,-D, C, E, G. In order to compare the feed-back energies in both cases with each other, merely the crosshatched areas in Fig. 1 are to be compared since the area G, A, D, H, E, G is a. portion of both areas. A comparison of the cross-hatched areas D, C, H, D, D and E, H, C, E, E shows that the feed-back energy increases with decreasing length of stroke.

In Fig. 1 the ratio man is relatively high.

' 1mm theTfeed-back energy decreases with increasing stroke, while for lower ratios pazm the feed-back energy increases with increasing stroke. The same result can be arithmetically calculated from the formula given below for the I of the length of the stroke without any special measures, su h as .the use of buffers, need be resorted to. Fig. 3 shows the diagram of a compressor, in

which the ratio changed. In the diagram this can be guessed from the fact that the surface C, H, E, E, C for the longer stroke up to C and the surface C, D, D, H, C for the shorter stroke up to C are equal.

, This ratio of pressures can be arithmetically calculated as follows: The area L1 of the surface F, D, C, E, F is given by the following formula (see Schiile, Technische Thermodynamik, 4th edition, page 114) In consequence thereof the feed-back energy, being the sum of L1 and L2, is equal to The feed-back energy L depends on the volume 04 so that generally it is not constant and depends on the length of the stroke. If however pzzpi is equal to it will be seen that the feed-back energy L has the value 121-91 and is independent of the length of the stroke. If it is assumed that the curves A, B and C, D are adiabatics, m is equalto 1.4 and the ratio pxpi is equal to 3.25. In the diagram shown in Fig. 3 pzzpi has this value and the cross-hatched surfaces have equal areas.

This invention relates to free piston motor compressors inwhich the compressor piston is larger than the motor piston, so that on the side of the compressor piston facing the motor pistonan annular surface remains over of a size equal to the difference in size of the two piston surfaces. It is known to keep this annular surface permanently in contact with the free atmosphere and it is further known to utilize the space of the compressor cylinder, which adjoins this annular surface, for the compression and conveying of gas The energy required therefor must be furnished by theamount of energy which is available'in the compressor space adjoining the outer surface of the compressor piston, when the freely moving mass passes through its return stroke. The same amount of energy must however also be resorted to for the compression of the charge of the motor cylinder. However the remainder of energy still available will in general be very small so that a high compression of the charge of the motor cylinders such as is required for the operation of free piston engines according to the Diesel process cannot be carried through any more.

According to the present invention I succeed charge, which remains substantially constant regardless of variations in the stroke of the freely movable masses by choosing the ratio of pressures in the compressor space adjoining the outer surface of the compressor piston smaller 7 than (wherein in is the coemcient of expansion) and.

location of the freely moving mass is alternately compressed and expanded. When the stroke of the freely moving masses rises, the amount of energy taken up by this quantity of gas during its compression will rise also and reversely it will diminish as the stroke of the freely moving masses drops. For the compression of the motor charge there is available the difl'erence between the energy liberated in the compressor space and the energy taken up in this annular space. Now, since for instance when the stroke of the freely moving masses drops, the energy liberated in the compressor space and similarly the energy taken up by the annular space become smaller,the difference between these two amounts'of energy will remain approximately constant, provided that the magnitudes of the spaces and pressures in the compressor space and in the annular space are so chosen, that the amounts of energy liberated in the compressor space and taken up in the annular space vary to the same extent during a variation of stroke.

In actual practice one finds that in most cases which occur practically, in order that the desired effect be obtained, the gas enclosed in the annular space need possess only a comparatively low medium pressure and only small variations of this pressure. This offers great advantages for the'operation of the machine since consequently also stresses arising from pressure and heat in the parts of the machine surrounding the annular space are only insignificant and since apart therefrom there is always available a resultant back-feed energy sumcing for a high compression of the charge of the motor cylinder, 1. e. for the Diesel operation of the motor part. Besides this, the magnitude of the feed-back energy, which becomes: available in thecompressor chamber,

can be varied within certain limits without influencing the compressor output by choosing the dimensions of the dead space of the compressor in proportion. For instance; while the output of the compressor remains unchanged, there corresponds to a larger dead space also a larger surface of the compressor piston than with a small dead space and consequently also an increase of the energy which becomes available in the compressor chamber during the return stroke. The magnitude'required in each individual case of the medium pressure and the required magnitude of the variations of pressure of the quantity of gas enclosed in the annular space,

can easily be obtained by suitably choosing the.

remains approximately constant also during variations of the stroke of the freely moving masses.

made exactly equal to Figs. 4 and 5 illustrate in a diagrammatical manner two embodlmentsof a machine according to the present invention. Similar parts are marked with similar numerals. Each of the two to each other two freely moving masses 2, l0,

each formed by a smaller motor piston 2 and the larger compressor rigidly connected with it. The end faces 25 of the motor pistons 2, which face each other, enclose between them the working chamber I of the motor. The outer end faces 26 form the end walls of the compressor spaces formed by the enlargements 5 and 6 of the casing. The annular rear surfaces 21 of the pistons III delimit spaces II, which communicate permanently with each other through a'conduit I2. The spaces II and conduit l2 are filled with a predetermined quantity of gas which permanently remains therein. 1 are the suction valves, 8 the pressure valves of the compressors, 9 is a device for feeding fuel to the motor cylinder, 3

are the inlet ports and 4 the outlet ports of the motor cylinder. These ports are uncovered and covered by a motor piston 2 of their own during its reciprocation. The exhaust gases pass from the cylinder through the ports 4 into the exhaust chamber 14'. In a machine of this kind thefeed-back energy will be constant and independent of the length of the stroke by choosing a pressure ration 1121p], equal to L mun-l only if the intermediate chambers H are evacuated and no gas pressure exists therein. In practice, however, it is very diflicult to accomplish this and in consequence thereof I provide means for maintaining the feed-back energy constant even if a gas pressure is exerted on the rear faces These 21 of the end portions ID of the pistons. means are based on the following reasoning:

Assuming that an invariable amount of gas is included in an intermediate chamber II, this amount of gas will be compressed during the return stroke of the piston. In consequence thereof energy is fed to this gas and this energy increases with the length of the stroke. It can only be supplied from the energy fed back by the gas in the compression chamber during the return stroke of the piston. In consequence thereof the ratio 172:1)1 should be chosen in such manner that the difference between the feedback energyand the energy stored during the return stroke in the intermediate chamber H is constant and independent of the length of the stroke in order to obtain a constant energy for compressing the charge in'the combustion chamber I. In order to accomplish this the ratiov pup; in the compression chamber'must not be mil-l but should be somewhat smaller so that the feedback energy increases with an increase of the stroke to an amount corresponding to the simulcompressor, provided that at the same time the dead space of the compressor is dimensioned accordingly. The dead space should be the larger, the larger the area of the piston surface 26.

The amount of energy to be stored in the intermediate chamber H during the return stroke of the piston substantially depends on the average pressure existing in the intermediate chamber. The necessary value of this. pressure and the desired variation thereof in dependency from the length of the stroke can easily be obtained by suitably dimensioning the volume of the chamber and the amount of gas enclosed therein.

In Fig. 4 the intermediate chambers II are connected with each other by a pipe l2, and by suitably dimensioning the volume of this pipe with respect to that of the chambers II the average pressure existing in the intermediate chambers l I can be adjusted. If for instance the pressure in the intermediate chambers l I and the pipe I2 is equal to the atmospheric pressure at the inner dead centre position of the pistons, this pressure will decrease during the outward stroke, but will rise to its former value during the return stroke of the freely moving masses. This change of pressure becomes the greater, the smaller the part of the spaces ii, l2, which is not passed by the piston It), more especially therefore the smaller the volume of the conduit l2. The higher the change of pressure, the greater is also (at a predetermined starting pressure) the amount of energy taken up by the gas contained in the spaces ll, I2 during the return stroke.

' Therefore if the space I I, I2 is made variable as to a clearance chamber 30, the upper-portion of its volume, without changing the quantity of gas enclosed therein, the pressure ratio in the compressor cylinders can also be varied within certain limits without renouncing to the advantage offered by the approximate constancy of the resulting feed back work. In order to change this volume at will the pipe 12 may be provided with which communicates with the interior of pipe l2 by means of a port 3|. with a threaded piston rod 33 extending through the end wall 34 of the cylinder 30. 35 is a nut engaging the threaded piston rod 33 and rotatably supported by the end wall 34. By rotating the nut as the position of the piston 32 and thus the volume of the space connecting the intermediatechambers H can be varied.

The maximum pressure existing in the chambers II and in the connection 12 may differ from atmospheric pressure. It may for instance be equal to the pressure in the scavenging air chamber l4. Preferably the intermediate chambers II and the connection I2 communicate with a space in which the maximum pressure exists, for instance with the atmosphere, or with the chamber H by means of a check valve opening into said space. A valve of this kind is indicated at 13 in Fig. 4. This valve prevents the pressure within the intermediate chambers II from exceeding atmospheric pressure even if the enlarged portions Ill 01 the pistons are not perfectly packed in the enlarged portions of the cylinder.

In Fig. 4 the pistons 2 are mechanically coupled with each other by means of rods 2| and 22 linked to a double armed lever 23 pivoted about its middle point. In consequence thereof the pistons reciprocate in opposite directions.

Fig. 5 shows an embodiment similar to that A piston 32 is provided II are connected to the chamber H to which the scavenging air is fed by a separate pump (not shown) in the direction of the arrow in Fig. 4 by a check valve l5 opening into the chamber I4. In consequence thereof the pressure in the intermediate chambers cannot exceed the pressure in the chamber l4. On the other hand the pipe l2 communicates with a space, for instance the atmosphere, in which exists the minimal pressure to bemaintained in the intermediate chambers H, by means of a valve l6 opening into the pipe l2. As soon as the lower limit of the pressure prevailing in the spaces I I, I2 starts dropping below-the atmospheric pressure, this valve [8 will open and admit air into the spaces II, l2, until the lower limit of pressure in these spaces again corresponds to atmospheric pressure. In consequence thereof the pressure in the intermediate chambers II is always maintained between atmospheric pressure and the pressure in the scavenging air chamber ll, even if the pistons 2 and their enlarged end portions in are not perfectly packed in their cylinders. If desired the pipe l2 may be provided with an extension and an adjustable piston therein similar to that shown Fig. 4.

I wish it to be understood that I do not desire to be limited to the exact details of construction shown and described, for obvious modifications will occur to a person skilled in the art.

I claim:-

1. A free piston engine comprising a cylinder having enlarged end portions, a pair of free pistons arranged in said cylinder, enlarged end portions or said piston being fitted into the end portions of said cylinder, a compression chamber and an intermediate chamber in each end portion of said cylinder, said chambers being separated from each other by an end portion of said'piston, a conduit connecting said intermediate chambers with each other and a body of gas in each compression chamber and in the intermediate chambers and in said conduit, the ratio of the pressures of each body of gas in a compression chamber at the outer and inner dead centre positions of said pistons being less than the average gas pressure in said intermediate chambers being so chosen that the difference of the total amounts of the energy fed by the gas to 'said pistons during their return stroke and of the energystored in said intermediate chambers during the return stroke is substantially constant.

2. A free piston engine comprising a cylinder having an enlarged end portion, a free piston nT-T m being the coeflicient of expansion of the gas, while the body of gas in said intermediate chamber has an average pressure such that the difference between the energy fed by the gas to said pistons during their return stroke and the energy stored in said intermediate chamber during the return stroke is-substantially constant.

3. A free piston engine comprising a cylinder having an enlarged end portion, a free piston arranged in said cylinder, an enlarged end portion of said pistonfitting into the enlarged end portion of said cylinder, a compression chamber and an intermediate chamber in said enlarged end portion of said cylinder, said chambers being separated from each other by said enlarged end portion of said piston, and a body of gas in said l5 compression chamber and in said intermediate chamber, respectively, the ratio of pressures of the body ofgas in said compression chamber at the outer and inner dead centre positions of said piston being less than 2o un-l m being the coeflcieht of expansion of the gas,

while the bodyof gas in said intermediate chamber has an average pressure such that the difierence between the energy fed by the gas to said pistons during their-return stroke and the energy stored in said intermediate chamber during the return strokeis substantially constant, and means for varying the gas "pressure in said intermediate chamber.

- 4. A free piston engine comprising a cylinder having an enlarged end portion, a. free piston arranged in said cylinder, an enlarged end portion of said piston fitting into the enlarged end portion of said cylinder, a compression chamber "and an intermediate chamber in said enlarged end portion of said cylinder, said chambers being separated from each other by said enlarged end portion 01 said :piston, and a body of gas in said compression chamber and in said intermediate chamber, respectively, the ratio of pressures of the body of gaszin'said compression chamber at the outer and inner dead centre positions of said piston beingilessithan In being the ccoeflicient of expansion of the gas, while the body ofgas in-said intermediate chamber has aniaverage pressure such that the difference between the energy ted by the gas to said pistons during-theinreturn stroke and the energy stored in saidiintermed iate chamber during the return strokeiis substantially constant, and means for varying ithe wolume of said intermediate chamber.

5. A free ipistoncengine comprising a cylinder having anenlarged end portion, a free piston arranged in said cylinder, an enlarged end portion of said ;piston fitting into the enlarged end portion otsaidccylindeL-a compression chamber and an intermediate chamber in said enlarged end portionaofr-said cylinder, said chambers being separated fromreach other by said enlarged end portion of said 'piston, and a body of gas in said compression chamber, and in said intermediate chamber, respectively, the ratio of pressures of the body oiigasi-in said compression chamber at the outer and inner dead centre positions oi'said piston being less than m being the coeflicient of expansion of the gas,

while the body of gas in said intermediate chamber has an average pressure such that the dinerence between the energy fed by the gas to said pistons during their return stroke and the energy stored in said intermediate chamber during the return stroke is substantially constant, and means for'maintaining the gas pressure in said intermediate chamber below a predetermined maximum.

6. A free piston engine comprising a cylinder having an enlarged end portion, a free piston arranged in said cylinder, an enlarged end portion of said piston fitting into the enlarged end portion of said cylinder, a compression chamber and an intermediate chamber in said enlarged end portion of said cylinder, said chambers being separated from each other by said enlarged end portion or said piston, and a body of gas in said compression chamber and in said intermediate chamber, respectively, the ratio of pressures of the body of gas in said compression chamber at the outer and innerdead centre positions of said piston beingless than m being the coeiiicient of expansion'ot the gas, while the body of gas in said intermediate chamber has an average pressure such that the dinerence between the energy fed by the gas to saidreturn stroke is substantially constant, and means for maintaining the gas pressure in said intermediate chamber above a predetermined minimum.

7. A free piston engine comprising a cylinder having an enlarged end portion, a free piston arranged-in said cylinder, an enlarged end portion of said piston fitting into the enlarged end portion or said cylinder, a compression chamber and an intermediate chamber in said enlarged end portion of said cylinder, said chambers being separated from each other by said enlarged end portion of said piston, and a body of gas in said compression chamber and in said intermediate chamber, respectively, the ratio of pressures of the body of gas in said compression chamber at the outerand inner dead centre positions of said piston being less than unl m being the coemcient of expansion of the gas, while the body of gas in said intermediate chamber has an average pressure such that the diiiference between the energy fed by the gas to said pistons during their return stroke and the energy 

